I don't like ironsight aim since it doesn't really add anything; you already have a crosshair so all it means is that they purposely made hip aim shit and you can see 50% less of the battlefield whenever you... want to see the battlefield
That being said, it has its places. In my opinion there are two different kinds of FPSes, first being the "realistic" shooter with cover-based combat, ironsight aim and low health. The setting doesn't have to be realistic at all (Borderlands) but the way you move is realistic; slow and careful making sure you can't be ambushed. These games use ironsights good since the movement penalty is actually part of the game mechanics, preventing people going guns blazing and keeping the tactical part in.
Then there's arena shooters, like quake and TF2, with high mobility and high health. I don't really know an example at the moment (did Tribes Ascend have it?) but in this ironsight is lethal to the gameplay. Arena shooters make or break on the movement, and being limited when shooting is like driving with the handbreak on. Arena shooters also tend to be more three-dimensional, while realistic shooters are 95% on the horizontal plane, with height advantage not being such an important factor, so in arena shooters losing view of the bottom half of your screen is horrible.
So in short ironsight is ok in realistic shooters but awful in arena shooters. I hate it but that's just my personal taste